The invention relates to a radiation-sensitive semiconductor device comprising a semiconductor body which has at least one radiation-sensitive diode with at least one pn junction between a first semiconductor region of a first conductivity type and a layer-shaped semiconductor zone of a second conductivity type opposite to the first conductivity type and having such a thickness and impurity concentration that the layer-shaped zone is depleted in the operating condition substantially throughout its thickness and over its complete surface area.
Radiation-sensitive semiconductor devices of the aforementioned kind are used inter alia for directly converting electromagnetic radiation, especially visible light and infrared radiation, into an electrical current or voltage. Such photodiodes are used, for example, in electrooptical communication techniques. Such semiconductor devices are also utilized in medical apparatus, such as an X-ray scanner which is provided with a scintillator of, for example, caesium diode which converts the X-ray radiation into radiation to which the photodiode is particularly sensitive. Additionally, such devices are used for detecting particle radiation, such as, for example, electron radiation.
A problem in such radiation detectors often is the high capacitance of the pn junction. Such a high capacitance adversely affects the high-frequency behavior of such a radiation detector and moreover gives rise to a poor signal-to-noise ratio especially at a weak signal (that is to say a low radiation intensity).
This capacitance is generally determined inter alia by the extent of the surface area of a diffusion region (mostly a p-type diffusion region in an n-type semiconductor body). However, the relevant surface area is preferably chosen to be as large as possible in order to obtain the highest possible radiation sensitivity of the detector and hence the highest possible photocurrent. In order to neutralize in part the high capacitance associated therewith, the surface area of the diffusion region can be slightly reduced in that this region is given, for example, a finger structure.
A semiconductor device of the kind mentioned above is known from Japanese Kokai No. 53-136987.
The capacitance of the radiation-sensitive pn junction shown therein is considerably reduced in that in the operating condition the layer-shaped semiconductor zone of the second conductivity type is fully depleted.
In such a device, problems may arise especially when the connection contacts both of the region of the first conductivity type and of the region of the second conductivity type are located on the side of the incident radiation. The contact metallizations, i.e. also those of the layer-shaped zone, are in fact preferably kept as small as possible in order to obtain the largest possible effective surface area of the radiation detector. The minority charge carriers generated by radiation in the depleted layer-shaped zone must reach the connection contact by diffusion, which adversely affects the speed of the radiation-sensitive semiconductor device. Especially in medical applications it is of importance that this speed is sufficiently high because this speed is a factor determinative of the exposure time for X-ray exposures and hence of the radiation dose to which the patient is subjected.